# Lead-acid battery charging fixed current stage specification

This post doesn't question the charging process or its theory. I seek to understand how to read the battery manufacturer's requirement of fixed current stage.

They prescribe a current to be held at specific value, let's call it I_fixed. I am assuming that to be a true DC current, which would be easy to comply with.

Now for reasons not important to discuss here, many chargers just rectify transformer output and give pulsating DC voltage. I control this with thyristors for each half-wave, thus current is pulsating. In order to measure the charging current, I decided to use RMS calculation from 256 samples I take each half-wave.

In short: Am I following correctly the manufacturer's specification to control the RMS current value to match their value? Is I_rms == I_fixed?

EDIT: Note that I have no choice with the hardware, I can only modify the software. The battery is lead-acid 36V 215Ah, voltage after thyristors is 36Vrms = 51Vpeak, pulsating 120 times for 60Hz AC. I control the pulse width for each half-wave pulse. The Irms I use now to charge goes from 0...21.5A

Pardon for hand-made chart... I noted this down during one test cycle. the current is what I calculated form the above described RMS current. Voltage is measured every 16th half-wave with no charge current present in 64 samples then averaged.

• Frankly, I think that you shouldn't be concerned about current discrepancy from RMS current and DC current specification. Rather the voltage could be the problem if you wish to have a this kind of charger continuously connected. Jun 17, 2019 at 18:00
• @Marko: My charger follows all 6 steps of recommended charging process. The transformer voltage is fixed (after thyristors 36Vrms, 51Vpeak pulsating) and I intelligently control each pulse width. Battery is lead-acid 36V. Every 16th pulse is off, during which the Vbattery is measured. Each charging pulse the Irms is calculated and compared to manufacturer's specification. My question was if I should measure some other current style and use that as comparison value. The battery voltage grows nicely as expected and acid never boils violently. Jun 18, 2019 at 15:16
• "Never boils violently", but it boils, so overcharging. It has to be determined whether you do want deliberately overcharge for limited time, thus having ventilation+ battery non sealed type. If it's sealed, then it can explode and you cannot add distilled water in it. You could draw a simplified schematics or a sketch of the charger and type of batteries, as well those specifications (link?). As said, the current is not so important, IMO. Jun 19, 2019 at 7:56
• It seems unlikely that anyone except experts who have investigated this exact point would be able to provide a definitive reply. The manufactures should be more able than most and www.batteryuniversity.com may be happy to actually consider the question as it seems reasonably likely that it has not been asked in this form. The most likely normal means of measuring current which results from phase controlled half cycles of current would be to measure the smoothed current as seen by a typical meter. This is probably closer to mean than RMS. ... Jun 20, 2019 at 11:57
• ... BUT it seems unlikely to matter too much. Your charge rate is nominally C/10 which is likely t o be well below the maximum allowed current specified by the manufacturer. Instantaneous currents in a phase controlled half cycle may be several times the mean or RMS values (and depend on cell impedance, wiring resistance and inductance, rectfier conduction angle and more. But even several times higher than C/10 is probably acceptable to the manufacturer so they are likely to be unlikely to have much to say. Jun 20, 2019 at 11:58

Lead acid 36V would have 18 cells, assuming that 12V lead acid has 6 cells.

Stage2:

The correct setting of the charge voltage limit is critical and ranges from 2.30V to 2.45V per cell. So 18 x 2.40 = 43,2V

Stage 3:

The recommended float voltage of most flooded lead acid batteries is 2.25V to 2.27V/cell., therefore 2.25 x 18 = 40.5V.

Based on recommendations you exceed all limits, but this recommendations are for common flooded lead acid battery. You should find what is the type of your battery: flooded, AGM, ...has additives: Calcium, Antimony,...

It is very possible that your voltage measuring method isn't very good. When you measure the open circuit voltage you get surface charge voltage, which can fool you. It would be necessary to load the battery with a known low resistance, for example 15 ohm and low Rdson MOSFET, for very short pulse duration and sample the voltage.

You can improve your circuit with adding a series choke and freewheeling diode, if you wish to have almost DC current. Source

• Marko, thank you for good information. Yes, my voltages are a bit above what they should be (plus I have a safety timeout), reason was exactly that my voltage measurement is not good. I see higher voltage than what I see afte a minute of quiet time. The load measurement sounds good to me. How long pulse you think I need to load it - can I manage within one half-wave? The current with 15R resistor will be roughly 3A, meaning the resistor would have to be 150W if permanently attached! Jun 21, 2019 at 14:35
• Short pulse, enough that your ADC and filters settles to steady state. 2W to 5W is more than enough. Jun 24, 2019 at 7:05

As you mentioned, even if we disregard the charging process or its theory (e.g. what effect a 100 times I_fixed pulse would have on the battery), the direct answer is: by using a pulsed charge process you are not following the specifications, assumed to be DC.

From your description I assume you can't control pulse width or amplitude, and just decide to turn on, or not, the thyristors at each half-wave. Current limitation for deeply discharged batteries would have to be provided by series resistors, which is what very simple constant (floating) voltage chargers do, if charging time or efficiency are not relevant.

Just to give an idea how the current limitation would affect your charge time, if you could control both pulse width and amplitude, the graph below varies between 1/100 of your period to full DC and assumes I_fixed = 10 A. The blue curve shows how high the pulses should be to transfer the same amount of charge to the battery and the red one shows the value to keep the same RMS current (that would heat the series resistance, for example).

In short: by not knowing the precise specifications for pulsed charging you would have to be conservative, which would affect your charging time.

• please read my comment above. I do control the pulse width intelligently each half-wave pulse (120 times for 60 Hz) and I read it actually helps the battery to have pulsating charge - but I have no choice changing this hardware but full control over software. I am only seeking an answer if the Irms calculation is the best I can do in my situation and use that to compare to battery spec. Jun 18, 2019 at 15:23